Nuclear fuel assembly

ABSTRACT

The present invention relates to a fuel assembly for a light-water reactor with a substantially square cross section wherein the fuel assembly comprises a plurality of fuel rods ( 4 ). The fuel rods ( 4 ) extend between a bottom tie plate ( 5 ) and a top tie plate ( 6 ) and a coolant is adapted to flow upwards through the fuel assembly. At least one of the top tie plates ( 6 ) or the bottom tie plates ( 5 ) comprises flow openings ( 22, 22   a,    22   b ) for the passage of the coolant and side supports ( 17 ) for supporting the fuel rods ( 4 ) in the lateral direction. The side supports ( 17 ) are designed in one and the same sheet-metal piece as the flow openings ( 22, 22   a,    22   b ) and the side supports ( 17 ) are folded substantially 90° in relation to the other structure of the top tie plate ( 6 ) or the bottom tie plate ( 5 ).

TECHNICAL FIELD

[0001] The present invention relates to a nuclear fuel assembly with asubstantially square cross section for a light water reactor comprisinga plurality of fuel rods extending between a top tie plate and a bottomtie plate.

BACKGROUND ART

[0002] In a nuclear reactor, moderated by means of light water, the fuelexists in the form of fuel rods, each of which contains a stack ofpellets of a nuclear fuel arranged in a cladding tube, a column ofextruded fuel cylinders or an uninterrupted column ofvibration-compacted powdered fuel. The cladding tube is normally made ofa zirconium-base alloy. A fuel bundle comprises a plurality of fuel rodsarranged in parallel with each other in a certain definite, normallysymmetrical pattern, a so-called lattice. The fuel rods are retained atthe top by a top tie plate and at the bottom by a bottom tie plate. Tokeep the fuel rods at a distance from each other and prevent them frombending or vibrating when the reactor is in operation, a plurality ofspacers are distributed along the fuel bundle in the longitudinaldirection. A fuel assembly comprises one or more fuel bundles, each oneextending along the main part of the length of the fuel assembly.Further, the fuel assembly comprises one or more centrally arrangedchannels which conduct non-boiling water through the fuel assembly toimprove the moderation in the central parts of the fuel assembly.

[0003] The core is immersed in water which serves both as coolant and asneutron moderator. During operation, the water flows from below andupwards through the fuel assembly, whereby, in a boiling waterlight-water reactor, part of the water is transformed into steam. Thepercentage of steam increases towards the top of the fuel assembly.Consequently, the coolant in the lower part of the fuel assemblyconsists of water whereas the coolant in the upper part of the fuelassembly consists both of steam and of water. This difference betweenthe upper and lower parts gives rise to special problems which must betaken into consideration when designing the fuel assembly.

[0004] It is therefore desirable to achieve a flexible fuel assembly fora boiling water reactor which, in a simple manner, may be given a shapein which the upper part of the fuel assembly differs from the lower partthereof. A fuel assembly for a boiling water reactor with theseproperties is shown in PCT/SE95/01478 (Int. Publ. No. WO 96/20483). Thisfuel assembly comprises a plurality of fuel units stacked on top of eachother, each comprising a plurality of fuel rods extending between a toptie plate and a bottom tie plate. The fuel units are surrounded by acommon fuel channel with a substantially square cross section. A fuelassembly of this type may in a simple manner be given y differentdesigns in its upper and lower parts.

[0005] Also in a light-water reactor of pressurized-water type, it maybe desirable to design the fuel assemblies such that each fuel assemblycomprises a plurality of fuel units stacked on top of each other. Asdescribed above, each one of the fuel units then comprises a pluralityof fuel rods extending between a top nozzle and a bottom nozzle. A fuelassembly for a pressurized-water reactor, however, comprises no fuelchannel.

[0006] One factor which must be taken into consideration when designingsuch fuel units for boiling water reactors is that the fuel rods must bekept at a distance from the fuel channel which surrounds them. In thesame way, the fuel rods in a fuel assembly for a pressurized-waterreactor must be kept at a distance from an adjacently located fuelassembly. Normally, these distances are achieved by designing spacersfor retaining and positioning the fuel rods in relation to each otherwith members therefor. In a fuel assembly for a boiling water reactor,these members make contact with the fuel channel and in a fuel assemblyfor a pressurized-water reactor, similar members make contact with aspacer provided in an adjacently located fuel assembly. In those caseswhere shorter fuel units of the above-mentioned type are used, spacersare not necessary, so the spaced relationship must be achieved in someother way.

[0007] It is also common to arrange some form of mixing vanes for thespacers. Since the need of spacers is eliminated, these must be achievedin some other way.

[0008] In CA 1 250 966, a fuel unit is shown in which the need ofspacers is eliminated. The fuel unit is intended to be used in a reactormoderated with heavy water with pressure tubes. The fuel units and thepressure tubes have a circular cross section. The fuel rods are arrangedbetween a pair of end plates. To ensure that the fuel rods are kept at adistance from the pressure tubes, the end plates are provided with beamsextending radially out from the respective end plate and in a directionparallel to the direction of flow and between the fuel rods. The beamsmake contact with the inner surface of the pressure tube and thus keepthe fuel rods at a distance therefrom. One disadvantage of thisembodiment is that it is complicated to manufacture since the beams mustbe joined to the end plates in some way, for example by welding orsoldering. Further, the end plates are designed to make tight contactwith each other or be welded together t o minimize the pressure drop ofthe coolant across the end plates. One disadvantage of this method isthat the fuel rods in this region are not given a possibility of growingdifferentially because of thermal expansion during operation of thereactor. The problem with differential growth, however, is not so greatin this type of reactors since the burnup of the fuel assemblies islimited.

[0009] The object of the present invention is to provide a fuel assemblywith a plurality of short fuel units comprising top and bottom tieplates, respectively, provided with members which are adapted to keepthe fuel rods at a distance from a surrounding, substantiallyrectangular, fuel channel or an adjacently arranged fuel assembly whichis simple to manufacture.

[0010] It is another object of the invention to provide a fuel assemblywith mixing vanes which are simple to achieve in the fuel assembly.

[0011] A further object of the invention is to provide a fuel assemblywith members which permit differential growth of the fuel rods.

SUMMARY OF THE INVENTION

[0012] The present invention relates to a fuel assembly with asubstantially square cross section for a light-water reactor comprisinga plurality of fuel rods arranged between a top tie plate and a bottomtie plate. The features which characterize this fuel assembly are statedin claim 1.

[0013] The fuel assembly comprises a plurality of fuel units, eachcomprising a plurality of fuel rods extending between a bottom tie plateand a top tie plate. The bottom tie plate and the top tie plate may beidentically designed. According to one aspect of the invention, thebottom tie plate and the top tie plate are manufactured by punching in asheet. The punched piece is provided with side supports which, prior tomounting to the fuel rods, are folded approximately 90° around an axisparallel to the plane of the top tie plate and the bottom tie plate,respectively. The task of the side supports, in a fuel assembly for aboiling water reactor, is to make contact with the fuel channel whichsurrounds the fuel rods to keep the fuel rods at a distance from thefuel channel. In a fuel assembly for a pressurized-water reactor, thetask of the side pieces is to make contact with adjacent bottom and toptie plates, respectively, in order thus to keep adjacently located fuelassemblies at a distance from each other.

[0014] The top tie plates and the bottom tie plates, respectively, aremade of sheet metal which is so thin that it is allowed to deformbecause of the differential growth of the fuel rods. It is suitable tochoose a thickness of the sheet of the order of magnitude of 0.7millimetres.

[0015] In one embodiment of the invention, the bottom tie plates arepreferably provided with tabs during the punching, which tabs, prior tomounting, may be folded into the desired angle and shape for achievingmixing vanes. The task of the mixing vanes is to mix the coolant flowingupwards through the fuel assembly to increase the cooling capacity ofthe coolant. Increased cooling capacity is desirable particularly in theupper part of the fuel assembly, where the coolant has a highertemperature.

[0016] In a further embodiment of the invention, in connection with orafter the punching, the bottom and top tie plates, respectively, areprovided with a folded cross section. The folded cross section resultsin two adjoining top and bottom tie plates making contact with eachother in portions formed between the fuel rods and in the creation of aspace between the top and bottom tie plates in those portions where thefuel rods are arranged. This space allows differential growth of thefuel rods because of thermal expansion by the top and bottom tie plates,respectively, being locally deformed into the space. In this way, therisk of fuel rods bending because of differential rod growth, hencepreventing the full effect of the coolant, is reduced.

[0017] The advantage of the fuel assembly according to the invention isthat it comprises top and bottom tie plates with side supports which aresimple to manufacture from a plane or profiled sheet. The top and bottomtie plates may also be provided in a simple manner with mixing vanes andthe desired cross section in one and the same work operation.

[0018] By forming the top and bottom tie plates, respectively, in thinsheet metal, only a small space is obtained between two fuel unitsstacked on top of each other. This is an advantage in that power peaks,which otherwise may arise in axial gaps without fissionable material,are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1a shows in a vertical section a fuel assembly of a boilingwater type with short fuel units.

[0020]FIG. 2 shows a section A-A of the fuel assembly in FIG. 1.

[0021]FIGS. 2a and 2 b show alternative embodiments in a sectioncorresponding to the section A-A of the fuel assembly in FIG. 1.

[0022]FIG. 3 shows in a vertical section a fuel assembly ofpressurized-water type with short fuel units.

[0023]FIG. 4 shows on the lefthand side of the figure a lower part andon the righthand side of the figure an upper part of a fuel assemblyaccording to FIG. 1, comprising top and bottom tie plates with sidesupports.

[0024]FIG. 5a shows a view from above of a top tie plate with sidesupports punched out in a plane sheet.

[0025]FIG. 5b shows a view from above of a bottom tie plate, punched outin a plane sheet, intended to be placed in the upper part of a fuelassembly. The bottom tie plate is provided with mixing vanes in one ofthe openings.

[0026]FIG. 5c shows in a view from the side how the side support of atop tie plate is inserted into the side support of a bottom tie platearranged above the top tie plate.

[0027]FIG. 5d shows a view from above of a bottom tie plate arrangedabove a top tie plate where one of the side supports of the top tieplate is formed to be folded in and be locked in the side support of thebottom tie plate.

[0028]FIG. 5e shows in a view from above an alternative embodiment ofthe side support of a top tie plate.

[0029]FIG. 5f shows a view from above of a top tie plate, punched out ina plane sheet, with side supports according to FIG. 5e.

[0030]FIG. 6a shows in a view from above mixing vanes punched out in atop tie plate and a bottom tie plate, respectively.

[0031]FIG. 6b shows in a view from above a top tie plate arranged in afuel unit with mixing vanes which are bent in relation to the plane ofthe top tie plate, towards the observer, into the desired shape.

[0032]FIG. 7a shows in a view from above mixing vanes arranged in a toptie plate around a space intended to be arranged in a region in a fuelunit where one fuel rod is missing.

[0033]FIG. 7b shows in a view from above mixing vanes arranged in a toptie plate around a space intended to be arranged in a region in a fuelunit where four fuel rods are missing.

[0034]FIG. 8 shows a top tie plate and a bottom tie plate with a foldedcross section. The top and bottom tie plates are adapted such that,between the fuel rods, they make contact with each other and, in thefuel rod positions, they are arranged at a distance from each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035]FIG. 1 shows a fuel assembly of a boiling water type comprising anupper handle 1, a lower end portion 2 and a plurality of fuel units 3stacked one above the other. Each fuel unit comprises a plurality offuel rods 4 arranged in parallel and in spaced relationship to eachother in a given lattice. Further, each fuel unit 3 comprises a bottomtie plate 5 and a top tie plate 6 for attachment of the fuel rods 4 intheir respective positions in the lattice. The fuel units 3 are stackedon top of each other in the longitudinal direction of the fuel assemblyand they are stacked in such a way that the top tie plate 6 in one fuelunit 3 is facing the bottom tie plate 5 in the next fuel unit 3 in thestack and such that the fuel rods 4 in all the fuel units 3 are parallelto one another. A fuel rod 4 contains fuel in the form of a stack offuel pellets 7 b of uranium arranged in a cladding tube 7 a. A coolantis adapted to flow from below and up through the fuel assembly.

[0036]FIG. 2 shows that a fuel assembly is enclosed in a fuel channel 8with a substantially square cross section. The fuel channel 8 isprovided with a hollow support member 9 of cruciform cross section,which is secured to the four walls of the fuel channel 8. In the centralchannel 14 formed of the support member 9, moderator water flows. Thefuel channel 8 surrounds four vertical channel-formed parts 10,so-called sub-channels, with an at least substantially square crosssection. The four sub-channels each comprises a stack of fuel units 3.Each fuel unit 3 comprises 24 fuel rods 4 arranged in a symmetrical 5x5lattice.

[0037] The fuel assembly in FIG. 2 comprises 10x10 fuel rod positions.By a fuel rod position is meant a position in the lattice. All the fuelrod positions in the lattice need not be occupied by fuel rods 4. Incertain fuel assemblies, a number of fuel rods 4 are replaced by one ora plurality of water channels. The introduction of a water channelchanges the number of fuel rods 4 but not the number of fuel rodpositions.

[0038]FIG. 2a shows an alternative embodiment of a fuel assemblyaccording to the invention. FIG. 2a shows a horizontal section throughthe fuel assembly which is provided with an internally arranged verticalchannel 14 a through which water is conducted in a vertical directionfrom below and upwards through the fuel assembly. The channel 14 a issurrounded by a tube 9 a with a substantially square cross section. Thefuel units 3 are kept in position by being fitted onto the tube whichsurrounds the vertical channel 14 a.

[0039]FIG. 2b shows an additional embodiment of a fuel assemblyaccording to the invention. The figure shows a horizontal sectionthrough the fuel assembly which is provided with two centrally arrangedvertical water rods 14 b through which water is conducted from below andupwards through the fuel assembly. The water rods 14 b have a diameterwhich is somewhat larger than the diameter of the fuel rods 4 and areformed with a substantially circular cross section. The fuel units 3 arekept in position by being fitted onto the water rods 14 b.

[0040]FIG. 3 shows a pressurized-water fuel assembly. In the same way asthe fuel assembly in FIG. 1, it comprises a plurality of fuel units 3stacked on top of each other. Each fuel unit 3 comprises a plurality offuel rods 4 arranged in parallel and in spaced relationship to eachother in a given lattice. Each fuel unit 3 further comprises a top tieplate 6 and a bottom tie plate 5 for attachment of the fuel rods 4 intheir respective positions in the lattice. The fuel units 3 are stackedon top of each other in the longitudinal direction of the fuel assemblyand they are stacked in such a way that the top tie plate 6 in one fuelunit 3 is facing the bottom tie plate 5 in the next fuel unit 3 in thestack, and such that the fuel rods 4 in all the fuel elements 3 areparallel to each other. A fuel rod 4 contains fissionable material inthe form of a stack of fuel pellets 7 b of uranium arranged in acladding tube 7 a. A coolant is adapted to flow from below and upwardsthrough the fuel assembly. A number of so-called control rod guide tubes4b are arranged extending through the whole fuel assembly. The controlrod guide tubes 4 b are intended to receive finger-shaped control rods(not shown) which are inserted into and withdrawn from, respectively,the guide tubes 4 b for the purpose of controlling the power of thenuclear reactor. The guide tubes extend between a top part 15 and abottom part 16. The top part 15 is arranged above the uppermost fuelunit 3 in the fuel assembly and the bottom part 16 is arranged below thelowermost fuel unit 3 in the fuel assembly. The fuel units 3 are kept inposition by being fitted onto the control rod guide tubes 4 b.

[0041]FIG. 4 shows a fuel assembly for a boiling water reactorcomprising top and bottom tie plates 6 and 5 with side supports 17. Onthe lefthand side, side supports 17 are shown which are arranged in alower part of the fuel assembly and on the righthand side in the figure,side supports 17 are shown which are arranged in an upper part. In thelower parts of the fuel assembly, it is suitable to arrange top andbottom tie plates 6, 5 with as low a flow resistance as possible. In theupper parts, on the other hand, it is suitable to arrange side supports17 which have the capacity to scrape off coolant flowing upwards throughthe fuel assembly along the walls of the fuel channel and to direct ittowards the centre of the fuel assembly. The bottom tie plates 5 aretherefore preferably formed with larger side supports 17 in the upperparts compared with the lower part of the fuel assembly.

[0042] In FIG. 4, a fuel rod 4 is indicated with a stack of fuelpellets. The fuel rods 4 are provided at their upper and lower ends,respectively, with a top plug 18 and a bottom plug 19, respectively,which are fixed to the top tie plate 6 and the bottom tie plate 5,respectively. In the upper end of the fuel rods 4, an axial gap 20 isarranged. The purpose of the axial gap 20 is to accumulate fissiongases, formed during operation, and thermal expansion of the column offuel pellets 7 b. The fuel pellets 7 b are shown with a cupped shape 21.The cup shape 21 makes it possible for part of the thermal expansion andthe formed fission gas to accumulate between the pellets 7 b, wherebythe axial gap 20 may be made correspondingly shorter in the axialdirection. It is desired as far as possible to avoid axial gaps 20 infuel assemblies since chemically aggressive environments may arise atthese assemblies.

[0043]FIG. 5a shows a view from above of a top tie plate 6, punched outin a substantially plane sheet, with side supports according to thelower part of the fuel assembly shown in FIG. 4. The top tie plate 6 isshown before the side supports 17 have been folded into their finalposition. Below the top tie plate 6, fuel rods 4 are indicated, with topplugs 18 arranged in the top tie plate 6. A bottom tie plate 5 intendedto be arranged so as to make contact with this top tie plate 6 may bedesigned in a manner corresponding to that of the shown top tie plate 6.

[0044] The top tie plate 6 in FIG. 5a is designed with a plurality offlow openings 22 disposed in an orthogonal pattern intended to betraversed by the coolant flowing upwards in the fuel assembly. Theseflow openings 22 are thus arranged substantially between the positionsof the fuel rods 4. Between the flow openings 22 there are arrangedsmaller openings 23 for fixing the fuel rods in top plugs 18. The sidesupports 17 are designed as substantially rectangular parts which areconnected via beams 24 to the structure of the rest of the top tie plate6. The side supports 17 are then folded so as to form an angle ofsubstantially 90° with the plane of the other structure. The sidesupports 17 may either be folded upwards or downwards in the axialdirection of the fuel assembly. Further, the side supports 17 areprovided with guiding tabs 25 arranged at the outer edge of the sidesupports 17. The task of the guiding tabs 25 is partly to guide the fuelassembly into a fuel channel 8 or between fuel assemblies ofpressurized-water type with spacers, partly to guide coolant flowingupwards through the assembly and guide it towards the centre of the fuelassembly. For this purpose, the guiding tabs 25 are folded around theedge of the side support 17 so as to form the desired angle, about 30°,therewith. The guiding tabs 25 are preferably arranged such that, inmounted position, they are arranged in the space between two fuel rods 4(see reference numeral 26 in FIG. 5e).

[0045]FIG. 5b shows an alternative embodiment of a bottom tie plate 5.The bottom tie plate 5 is intended to be placed in the upper part of afuel assembly and is provided with more and larger supports 17 than whatis shown in FIG. 5a and with more guiding tabs 25 for the purpose ofachieving better guiding of coolant to the central parts of the fuelassembly. To increase the stability of the bottom tie plate 5, the sidesupports 17 are designed so that, in folded position, they can be joinedto each other by means of, for example, welding or soldering (seereference numeral 27). In addition, the bottom tie plate 5 is providedwith mixing vanes 28 in one of the flow openings 22. The mixing vanes 28are shown in unfolded state but are folded so as to form a suitableangle with the plane of the rest of the structure. The object of themixing vanes 28 is to mix the coolant flowing upwards through theassembly to increase its cooling capacity and reduce the risk of thetemperature around a fuel rod 4 becoming so high that the coolant leavesthe fuel rod, in which case so-called dryout may arise. The risk ofdryout is greatest in the upper part of the fuel assembly, and thereforethe bottom plates 5 intended to be arranged in this part are suitablyprovided with mixing vanes 28 in one or more flow openings 22.

[0046]FIGS. 1, 3 and 4 show that two top tie and bottom tie plates 6 and5 arranged adjacent to each other are provided with side supports 17folded in different directions, that is, the side support of the bottomtie plate is folded upwards and the side support of the top tie plate 6is folded downwards. FIG. 5c shows an embodiment in which the sidesupports 17 of both the top tie plate 6 and the bottom tie plate 5 arefolded upwards in the fuel assembly. In addition, the side support 17 ofthe top tie plate 6 is arranged inserted into and fixed to the sidesupport 17 of the bottom tie plate 5 for joining or guiding of two fuelunits 3 stacked on top of each other. FIG. 5d shows the top tie plate 6and the bottom tie plate 5, respectively, according to FIG. 5c inunfolded form. The outer side support 17 shown in FIG. 5d thus belongsto the top tie plate 6 arranged below the bottom tie plate 5. Whenstacking the fuel units 3 on top of each other, the side supports 17 arefitted into each other and fixed in such a way that they may be detachedfrom each other again.

[0047]FIG. 5e shows an alternative embodiment of a top tie plate 6 withside supports which are considerably smaller than those shown in FIGS.5a-5 d. The side supports 17 are substantially tapering in an outwarddirection and are provided at their respective ends with guiding tabs25. In FIG. 5f, where the top tie plate 6 is shown unfolded in a viewfrom above, it is clear that the rest of the structure has also beengiven a more open shape than the structures shown in FIGS. 4 and 5a-5 d.The more open structure has been achieved by forming no beams 29 betweensome of the flow openings 22. A top tie plate 6 with this design givesrise to a very low flow resistance and hence a low pressure drop acrossthe fuel assembly. A bottom tie plate 5 may, of course, be formed in thesame way as the top tie plate 6 described here.

[0048]FIG. 5f further shows two different embodiments of the design ofthe side supports 17. The design at reference numeral 33 a is preferablefrom the point of view of pressure drop in that the structure is open.The design at reference numeral 33 b gives a stiffer construction of theside supports. In an advantageous embodiment, the side supports 17 arefolded with as small a radius as is possible in practice, which alsocontributes to increased stiffness.

[0049]FIG. 6a shows part of a top and bottom tie plate 6 and 5,respectively, with punched-out and still unfolded mixing vanes 28 in theflow openings 22. The mixing vanes 28 are tapering in an outwarddirection and extend from the structure and into the flow opening 22.FIG. 6b shows the same mixing vanes 27 folded up from the plane of therest of the structure, towards the observer. The arrows in FIG. 6bindicate the rotation and mixing achieved by the mixing vanes 28 in theflow which passes the flow opening 22. In FIG. 6b, also the fuel rods 4arranged below the plate 5, 6 are indicated.

[0050]FIG. 7a shows part of a top and bottom tie plate 6 and 5,respectively, with punched-out and folded mixing vanes 28 in an enlargedflow opening 22 a. The flow openings 22 may be enlarged in the eventthat fuel rods 4 are missing in some of the fuel rod positions of thefuel unit 3. FIG. 7a shows an enlarged flow opening 22 a arranged abovean empty fuel rod position. The flow opening 22 a may then be providedwith mixing vanes 28 according to FIG. 7a. FIG. 7b shows a correspondingenlarged flow opening 22 b arranged above four empty fuel rod positions.The arrows in FIGS. 7a, 7 b indicate the mixing of the flow caused bythe mixing vanes 28 projecting from the structure.

[0051]FIG. 8 shows a top and bottom tie plate 6 and 5 with a foldedcross section. The top and bottom tie plates 6 and 5 are arranged so asto make contact with each other between the fuel rods 4 (see referencenumeral 30) and are arranged in spaced relationship to each other in thefuel rod positions (see reference numeral 31). The folded top and bottomtie plates 6 and 5 should be formed with flattened angles to preventfuel rods 4 from moving laterally within the fuel unit 3 as a result ofthe top and bottom tie plates 6 and 5 being deformed when the fuel rod 4grows axially.

[0052]FIG. 4 shows that the fuel rods 4 are provided with top and bottomplugs 18 and 19, respectively, with lugs which are inserted into andfixed to the top and bottom tie plates 6 and 5, respectively. FIG. 8shows an alternative embodiment where the top and bottom tie plates 6and 5, respectively, are instead joined together by means of laserwelding. The fuel rod 4 is then placed tightly against the bottom andtop tie plates 5 and 6, respectively, whereafter the materials in theplug and the plate 5, 6 are fused by means of laser technique (seereference numeral 32) or by means of resistance welding.

[0053]FIGS. 4 and 5 indicate top and bottom tie plates 6 and 5 for afuel assembly of boiling water type but the same principles are, ofcourse, applicable to fuel assemblies of pressurized-water type. In fuelassemblies of pressurized-water type, however, the top and bottom tieplates 6 and 5, respectively, are adapted to make contact withcorresponding top and bottom tie plates 6 and 5, respectively, inadjacently located fuel units 3. All the embodiments of the shown toptie plates are also applicable to bottom tie plates.

1. A fuel assembly for a light-water reactor with a substantially squarecross section comprising a plurality of fuel rods (4) extending betweena bottom tie plate (5) and a top tie plate (6) and wherein a coolant isadapted to flow upwards through the fuel assembly, characterized in thatthe top tie plate (6) or the bottom tie plate (5) comprises flowopenings (22, 22 a, 22 b) for the passage of the coolant and sidesupports (17) for supporting the fuel rods (4) in the lateral directionwherein the side supports (17) are formed in one and the samesheet-metal piece as the flow openings (22, 22 a, 22 b) and wherein theside supports (17) are folded substantially 90° in relation to the restof the structure of the top tie plate (6) or the bottom tie plate (5).2. A fuel assembly according to claim 1 , characterized in that the sidesupports (17) are made with a substantially rectangular cross section.3. A fuel assembly according to claim 1 or 2 , characterized in that theside supports (17) are formed such that, in folded-up state, they may bejoined (27) to an adjacent side support (17) arranged in the same toptie plate (6) and bottom tie plate (5), respectively.
 4. A fuel assemblyaccording to claim 1 , 2 or 3, characterized in that side supports (17)are adapted such that a side support (17) in a top tie plate (6) may befitted into a bottom tie plate arranged adjacent to said top tie plate(6), or vice versa, such that the side supports (17) thereof are fixedto each other in a detachable manner.
 5. A fuel assembly according toany of the preceding claims, characterized in that the side supports(17) are made with a substantially tapering shape.
 6. A fuel assemblyaccording to any of the preceding claims, characterized in that guidingtabs (25) are arranged in the outer ends of the side supports (17), saidtabs being folded in relation to the plane of the side support (17) andtowards the central part of the top (6) or bottom (5) tie plate.
 7. Afuel assembly for a light-water reactor with a substantially squarecross section comprising a plurality of fuel rods (4) extending betweena bottom tie plate (5) and a top tie plate (6) and wherein a coolant isadapted to flow upwards through the fuel assembly, characterized in thatthe top tie plate (6) or the bottom tie plate (5) is made in onesheet-metal piece with a plurality of flow openings (22, 22 a, 22 b) andthat, in the same sheet, mixing vanes (28) are arranged, punched out inthe flow openings (22, 22 a, 22 b) and folded in relation to the rest ofthe plane of the top (6) or bottom (5) tie plate for mixing the coolantflowing through the flow opening (22, 22 a, 22 b).
 8. A fuel assemblyfor a light-water reactor with a substantially square cross sectioncomprising a plurality of fuel rods (4) extending between a bottom tieplate (5) and a top tie plate (6) and wherein a coolant is adapted toflow upwards through the fuel assembly, characterized in that the toptie plate (6) or the bottom tie plate (5) is made with a wavy crosssection such that a top tie plate (6) arranged adjacent to a bottom tieplate (5) makes contact therewith in portions (30) surrounding flowopenings (22) and that, between these portions, they are arranged inspaced relationship to each other.
 9. A fuel assembly, characterized inthat it comprises a plurality of fuel units (3) stacked on top of eachother, wherein at least one fuel unit (3) comprises a top tie plate (6)or a bottom tie plate (5) according to any of the preceding claims.